Microlithography (also referred to as photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as deep ultraviolet (DUV) or vacuum ultraviolet (VUV) light. Next, the wafer with the photoresist on top is exposed to projection light in a projection exposure apparatus. The apparatus projects a mask containing a pattern onto the photoresist so that the latter is only exposed at certain locations which are determined by the mask pattern. After the exposure the photoresist is developed to produce an image corresponding to the mask pattern. Then an etch process transfers the pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component.
A projection exposure apparatus typically includes a light source, an illumination system that illuminates the mask with projection light produced by the light source, a mask stage for aligning the mask, a projection objective and a wafer alignment stage for aligning the wafer coated with the photoresist. The illumination system illuminates a field on the mask that may have the shape of a rectangular or curved slit, for example.
In current projection exposure apparatus a distinction can be made between two different types of apparatus. In one type each target portion on the wafer is irradiated by exposing the entire mask pattern onto the target portion in one go. Such an apparatus is commonly referred to as a wafer stepper. In the other type of apparatus, which is commonly referred to as a step-and-scan apparatus or scanner, each target portion is irradiated by progressively scanning the mask pattern under the projection beam along a scan direction while synchronously moving the substrate parallel or anti-parallel to this direction. The ratio of the velocity of the wafer and the velocity of the mask is equal to the magnification of the projection objective, which is usually smaller than 1, for example 1:4.
It is to be understood that the term “mask” (or reticle) is to be interpreted broadly as a patterning mechanism. Commonly used masks contain opaque or reflective patterns and may be of the binary, alternating phase-shift, attenuated phase-shift or various hybrid mask type, for example. However, there are also active masks, e.g. masks realized as a programmable mirror array. Also programmable LCD arrays may be used as active masks.
As the technology for manufacturing microstructured devices advances, there are ever increasing demands also on the illumination system. Ideally, the illumination system illuminates each point of the illumination field on the mask with projection light having a well defined spatial and angular irradiance distribution. The term angular irradiance distribution describes how the total light energy of a light bundle, which converges towards a particular point in the mask plane, is distributed among the various directions of the rays that constitute the light bundle.
The angular irradiance distribution of the projection light impinging on the mask is usually adapted to the kind of pattern to be projected onto the photoresist. Often the angular irradiance distribution depends on the size, orientation and pitch of the features contained in the pattern. The most commonly used angular irradiance distributions of projection light are referred to as conventional, annular, dipole and quadrupole illumination settings. These terms refer to the irradiance distribution in a pupil plane of the illumination system. With an annular illumination setting, for example, only an annular region is illuminated in the pupil plane. Thus there is only a small range of angles present in the angular irradiance distribution of the projection light, and all light rays impinge obliquely with similar angles onto the mask.
Different mechanisms are known in the art to modify the angular irradiance distribution of the projection light in the mask plane so as to achieve the desired illumination setting. In the simplest case a stop (diaphragm) including one or more apertures is positioned in a pupil plane of the illumination system. Since locations in a pupil plane translate into angles in a Fourier related field plane such as the mask plane, the size, shape and location of the aperture(s) in the pupil plane determines the angular irradiance distributions in the mask plane. However, any change of the illumination setting involves a replacement of the stop. This makes it difficult to finely adjust the illumination setting, because this would involve a very large number of stops that have aperture(s) with slightly different sizes, shapes or locations. Furthermore, the use of stops inevitably results in light losses and thus in a reduced throughput of the apparatus.
Many common illumination systems therefore include adjustable elements that make it possible, at least to a certain extent, to continuously vary the illumination of the pupil plane. Many illumination systems use an exchangeable diffractive optical element to produce a desired spatial irradiance distribution in the pupil plane. If zoom optics and a pair of axicon elements are provided between the diffractive optical element and the pupil plane, it is possible to adjust this spatial irradiance distribution.
Recently it has been proposed to use mirror arrays that illuminate the pupil plane. In EP 1 262 836 A1 the mirror array is realized as a micro-electromechanical system (MEMS) including more than 1000 microscopic mirrors. Each of the mirrors can be tilted in two different planes perpendicular to each other. Thus radiation incident on such a mirror device can be reflected into (substantially) any desired direction of a hemisphere. A condenser lens arranged between the mirror array and the pupil plane translates the reflection angles produced by the mirrors into locations in the pupil plane. This known illumination system makes it possible to illuminate the pupil plane with a plurality of spots, wherein each spot is associated with one particular microscopic mirror and is freely movable across the pupil plane by tilting this mirror.
Similar illumination systems are known from US 2006/0087634 A1, U.S. Pat. No. 7,061,582 B2, WO 2005/026843 A2 and WO 2010/006687 A1. US 2010/0157269 A1 discloses an illumination system in which an array of micromirrors is directly imaged on the mask.
As mentioned further above, it is usually desired to illuminate, at least after scan integration, all points on the mask with the same irradiance and angular irradiance distribution. If points on the mask are illuminated with different irradiances, this usually results in undesired variations of the critical dimension (CD) on wafer level. For example, in the presence of irradiance variations the image of a line having a uniform width on the light sensitive surface may also have irradiance variations along its length. Because of the fixed exposure threshold of the resist, such irradiance variations directly translate into widths variations of a structure that shall be defined by the image of the line.
If the angular irradiance distribution varies over the illumination field on the mask, this also has a negative impact on the quality of the image that is produced on the light sensitive surface. For example, if the angular irradiance distribution is not perfectly balanced, i.e more light impinges from one side on a mask point than from the opposite side, the conjugate image point on the light sensitive surface will be laterally shifted if the light sensitive surface is not perfectly arranged in the focal plane of the projection objective. For modifying the spatial irradiance distribution in the illumination field U.S. Pat. No. 6,404,499 A and US 2006/0244941 A1 propose mechanical devices that include two opposing arrays of opaque finger-like stop elements that are arranged side by side and aligned parallel to the scan direction. Each pair of mutually opposing stop elements can be displaced along the scan direction so that the distance between the opposing ends of the stop elements is varied. If this device is arranged in a field plane of the illumination system that is imaged by an objective on the mask, it is possible to produce a slit-shaped illumination field whose width along the scan direction may vary along the cross-scan direction. Since the irradiance is integrated during each scan cycle, the integrated irradiance (sometimes also referred to as illumination close) can be finely adjusted for a plurality of cross-scan positions in the illumination field.
However, these devices are mechanically very complex and expensive. This is also due to the fact that these devices have to be arranged in or very close to a field plane in which usually the blades of a movable field stop is arranged.
Adjusting the angular irradiance distribution in a field dependent manner is more difficult. This is mainly because the spatial irradiance distribution is only a function of the spatial coordinates x, y, whereas the angular irradiance distribution also depends on the direction of incidence given by a pair of angles α, β.
WO 2012/100791 A1 discloses an illumination system in which a first mirror array is used to produce a desired irradiance distribution in the pupil plane of the illumination system. In close proximity to the pupil plane an optical integrator is arranged that has a plurality of light entrance facets. Thus images of the light entrance facets are superimposed on the mask. The light spots produced by the mirror array have an area that is at least five times smaller than the total area of the light entrance facets. Thus it is possible to produce variable light patterns on the light entrance facets. In this manner different angular irradiance distributions can be produced on different portions of the illumination field. It is thus possible, for example, to produce an X dipole and a Y dipole illumination setting at a given time in the illumination field.
In order to ensure that the portions with different illumination settings are sharply delimited, it is proposed to use a second mirror array configured as a digital mirror device (DMD). This second mirror array is illuminated by the first mirror array and is imaged on the light entrance facets by an objective. By bringing larger groups of micromirrors of the second mirror array in an “off”-state, it is possible to produce irradiance distributions on the light entrance facets that have sharp boundaries.
However, it turned out that it is difficult to produce so many and so small freely movable light spots with the first mirror array. Furthermore, this prior art illumination system is mainly concerned with producing completely different illumination settings at different portions in the illumination field. For that reason the light entrance facets are usually not completely, but only partially illuminated.
In prior art illumination systems of the scanner type an adjustable field stop including movable blades opens and closes the slit-like illumination field at the beginning and the end of each scan cycle, respectively. This ensures that only the desired portions on the mask are projected on the light sensitive surface. The adjustable field stop has to perform its task reliably and with the highest accuracy. For that reason it is usually a mechanically complex and expensive device. Furthermore, the blades of the adjustable field stop have to be imaged on the mask with the help of a large objective. This objective involves a lot of space and significantly adds to the total manufacturing costs of the illumination system. WO 2010/006687 A1 discloses an illumination system that is similar to the one disclosed in WO 2012/100791 A1 that has been mentioned above. Here the variable light patterns on the light entrance facets are not used to modify a field dependence of the angular light distribution, but to take over the function of the movable blades of the adjustable field stop. However, also in this prior art illumination system very small light spots have to be produced on the light entrance facets. These spots are produced by a mirror array at positions that can be varied by changing a deflection angle produced by the mirrors.